The present invention relates to a method for preventing premature curing of silicone compositions and to compositions made thereby. More particularly, the present invention substantially eliminates premature curing of silicon hydride containing silicone compositions caused by the presence of residual Lewis acids by adding thereto an effective amount of Lewis base.
Curable silicone compositions comprising (a) a vinyl-containing base polymer, (b) an organohydrogenpolysiloxane crosslinking agent and (c) a precious metal or precious metal containing hydrosilylation catalyst as well known in the art, for example, as described in U.S. Pat. No. 3,284,406 to Nelson and U.S. Pat. No. 3,436,366 to Modic, both of which are incorporated by reference into the present disclosure.
More recently a great deal of interest has been shown in fluorosilicone compositions. Such fluorosilicone compositions are based on the same addition cure chemistry as U.S. Pat. Nos. 3,284,406 and 3,436,366, however, by including perfluoroalkylsiloxy units in the vinyl-containing base polymer excellent solvent resistance was obtained. Examples of fluorosilicone compositions can be found in U.S. Pat. No. 4,029,629 to Jeram, U.S. Pat. No.4,041,010 to Jeram, U.S. pat. application Ser. No. 414,226, filed Sept. 2, 1982, now U.S. Pat. No. 4,529,752, and U.S. pat. application Ser. No. 443,545, filed Nov. 22, 1982, now U.S. Pat. No. 4,585,848 all of which are assigned to the same assignee as the present invention and are incorporated herein by reference.
While any of the foregoing compositions can be prepared by merely mixing the various components together, it is often most convenient to prepare these compositions in two separate portions or packages which are combined at the time the compositions are to be converted to the solid, cured, elastic state. In the case of such a two component formulation it is convenient to include in the first package the vinyl-containing polysiloxane, the precious metal catalyst, and some or all of a finely divided filler. The second package contains as its sole essential ingredient the organohydrogenpolysiloxane crosslinking agent, but as a matter of convenience the second package can also contain a portion of the vinyl-containing base polymer and a portion of the finely divided filler.
The finely divided filler typically includes at least some fumed silica or treated fumed silica, both of which are known to contain silanol groups. Furthermore, it is not uncommon to include silanol functional plasticizing agents in silicone compositions, especially condensation curable room temperature vulcanizing silicone compositions.
On occasion the aforesaid curable silicone compositions have exhibited premature curing. Such premature curing has been both dramatic, for example, curing in the doughmixer, and subtle, such as gradual increasing of the viscosity of the composition to a point where processing was precluded. This premature curing was particularly noticeable in the aforesaid second component of two component addition curable compositions. Since the second component generally contains some vinyl-containing polymer it was thought that such premature curing was due to precious metal catalyst contamination.
The present applicants have now discovered that the true cause of premature curing is a Lewis acid catalyzed condensation reaction involving the organohydrogenpolysiloxane and silanol containing components such as the filter and/or plasticizer. The Lewis acid catalyst is believed to be generated in situ, for example, through the reaction of trace amounts of HCl and/or chlorosilanes with a group VIII metal used in a coloring agent of the second component.